Genesis 25:21 says Isaac prayed to the Lord on behalf of his wife, because she was barren; and the Lord answered him and Rebekah his wife conceived.
Isaac saw a need and took it to God in prayer. God heard. God answered Isaac's prayer, and something happened. God is good and I am sure there was much rejoicing in Isaac's camp when Rebekah discovered she was pregnant.
There is an interesting detail about this prayer and answer. Rebekah was barren so Isaac prayed. Isaac knew that God had promised many descendants. He trusted that God would provide children. So he prayed.
And he prayed for twenty years. Twenty years with no children passed before the birth of Esau and Jacob. Can you imagine the heartbreak as each year went by?
It can be difficult to keep praying when it seems like God is not listening. Sometimes it can seem like God is saying No when actually He is saying Wait - Have Faith - Trust Me - Keep Praying - I'm Listening.